The present invention is used particularly advantageously, albeit without any kind of limitation, in ultra-narrowband wireless communication systems. The term “ultra-narrowband” (UNB) is understood to mean that the instantaneous frequency spectrum of the radio signals transmitted by the terminals has a frequency width of less than one kilohertz.
Such UNB wireless communication systems are particularly suitable for M2M (machine-to-machine) applications or for applications related to the Internet of things (IoT).
In such a UNB wireless communication system, data exchanges are essentially unidirectional, in this case over an uplink between terminals and an access network of said system.
The terminals transmit uplink messages that are collected by base stations of the access network, without having to associate themselves beforehand with one or more base stations of the access network. Stated otherwise, the uplink messages transmitted by a terminal are not intended for one specific base station of the access network, and the terminal transmits its uplink messages under the assumption that they will be able to be received by at least one base station. Such arrangements are advantageous in that the terminal does not need to make regular measurements, which are intensive particularly from the point of view of power consumption, in order to determine the most appropriate base station to receive its uplink messages. The complexity is on the access network, which needs to be capable of receiving uplink messages that can be transmitted at arbitrary times and at arbitrary center frequencies. Each base station of the access network receives uplink messages from the various terminals that are within its range.
Such a mode of operation, in which data exchanges are essentially unidirectional, is entirely satisfactory for many applications, such as, for example, remote reading of gas, water and electricity meters, remote surveillance of buildings or houses, etc.
In some applications, however, it may be advantageous to be able also to exchange data in the other direction, namely over a downlink from the access network to the terminals, for example in order to reconfigure a terminal and/or control an actuator connected to said terminal. However, it is necessary to provide such a capability while limiting the impact on the power consumption of the terminals.
U.S. Pat. No. 6,130,914 describes an exemplary bidirectional UNB wireless communication system making it possible to limit the impact on the power consumption of the terminals.
Specifically, in U.S. Pat. No. 6,130,914, a downlink message is transmitted to a terminal within a listening window that is predetermined with respect to an uplink message transmitted by said terminal. More particularly, after having transmitted an uplink message, a terminal switches to a standby (or power-saving) mode for a standby window of predetermined duration. At the end of said standby window, the terminal leaves standby mode in order to listen to the downlink during a listening window of limited duration, while awaiting a downlink message transmitted by a base station.
On the access network side, the listening windows of the various terminals may be determined on the basis of uplink messages received by these terminals, and the access network must organize the transmission of the downlink messages so that they may be received, by the corresponding terminals, within the respective listening windows of said terminals.
Because the terminals listen to the downlink only for predetermined listening windows, the additional power consumption required for the reception of downlink messages is limited, and said terminals may be in standby mode most of the time. Furthermore, because they do not have to transmit and receive simultaneously, such terminals may be half-duplex terminals and may therefore be inexpensive to manufacture.
Additionally, since the terminals are not previously associated with particular base stations, the access network does not know which terminals are within range of the various base stations. This problem is also solved since a terminal listens only after having transmitted an uplink message, such that it is sufficient to use a base station having received said uplink message for the purpose of transmitting the downlink message to this terminal.
However, for such bidirectional UNB wireless communication systems, it is also desirable, in particular for reasons related to the cost of deployment of the access network, to use half-duplex base stations, i.e. base stations that are able to receive uplink messages and transmit downlink messages, but not simultaneously. In such a case, it is understood that a base station that switches to a transmission mode, in order to transmit a downlink message to a terminal, is no longer available for receiving uplink messages transmitted by other terminals, such that numerous uplink messages may thus be missed.